An antenna is conventionally configured to be capable of receiving or transmitting only a signal of a desired frequency band via a filter such as a band pass filter. Conventionally, the filter individually manufactured has been externally connected to an antenna for use. Recently, an antenna with built-in filter wherein an antenna and a filter are integrated has been developed for eliminating a troublesome work of matching an antenna and a filter or for meeting a demand of downsizing an electronic device.
As shown by a partially perspective explanatory view and its sectional explanatory view of FIGS. 7A and 7B illustrating one example of a state where an antenna with built-in filter 85 is mounted on a circuit board 86, the antenna with built-in filter 85 as described above has a laminated dielectric block 83 obtained by laminating dielectric sheets having a conductive pattern formed thereon for forming a capacitor or inductor that composes a filter and by sintering the resultant. A radiation element 81 is formed on the front face of the laminated dielectric block 83. One electrode of the filter is electrically connected to the radiation element 81, while the other electrode is connected to a feeding terminal electrode 84 provided at the outer face of the laminated dielectric block 83 so as to be able to be connected to a transmit-receive circuit that is an external circuit.
As shown in FIG. 7B in which the sectional explanatory view of FIG. 7A is illustrated, in the conventional antenna with built-in filter of this type, a wiring film 834 is extended to an end section to be drawn out from the other electrode of the filter 82 made of a conductive pattern on which an inductor or capacitor is formed, and after forming the laminated dielectric block 83, the feeding terminal electrode 84 is connected to the wiring film 834 exposed to its side face, whereby the feeding terminal electrode 84 is formed so as to go up to the back face (mounting face of the circuit board 86) from its side face. This feeding terminal electrode 84 is directly connected to a feeder 862 of the circuit board 86 by a soldering.
As described above, in the conventional antenna with built-in filter, the other electrode of the filter is drawn to the side face of the dielectric block 83 and the feeding terminal electrode 84 is provided from the side face of the dielectric block 83 to the mounting face (the face opposing to the circuit board on which the antenna is mounted, i.e., back face) so as to be connected to the drawn wiring film 834. Therefore, the case is likely to occur where radio wave received and transmitted by the radiation element 81 is directly picked up by the feeding terminal electrode 84 on the side face of the dielectric block. Further, an antenna is recently configured to be capable of receiving and transmitting a signal of two or more frequency bands by a cellular phone, i.e., an antenna is recently configured to not only receive and transmit a signal for cellular but also receive a signal of GPS (Global Positioning System) or to be used for Blue-tooth antenna for wireless LAN by a cellular phone. Therefore, signals of two or more frequency bands are likely to be fed to the feeding terminal electrode 84.
On the other hand, formed on the circuit board 86 having the antenna with built-in filter 85 mounted thereon are electronic circuits such as a receiving circuit or transmitting circuit connected to the feeder 862, in which a low noise amplifier not shown is included on the receiving side. In the case where reception and transmission are performed in two or more frequency bands, these electronic circuits are formed for each frequency band. The feeding terminal electrode and these electronic circuits are electromagnetically coupled, with the result that the noise or a signal of the other frequency band picked up by the feeding terminal electrode interferes directly with the circuits on the circuit board, thereby entailing a problem of deteriorating (increasing noise) isolation property (meaning that the coupling between each other is small) or reducing transmit-receive characteristic.
The present invention is accomplished to solve the aforementioned problem, and aims to provide an antenna with built-in filter that is designed to downsize by integrating a filter, made by laminating dielectric sheets, and a radiation element, wherein a mutual interference is not caused between an electronic circuit on a circuit board and a feeding terminal electrode of the antenna even if the antenna is directly mounted on the circuit board, thereby being capable of enhancing isolation property and improving transmit-receive characteristic.
Another object of the present invention is to provide a mounting structure of an antenna with built-in filter mounted on a circuit board so as to be suitable for preventing a mutual interference between the antenna and an electronic circuit, when this antenna with built-in filter is mounted on the circuit board.